1. Field of the Invention
The invention relates to a semiconductor optical amplifier and a semiconductor laser, and more particularly to a semiconductor optical amplifier transmitting improved saturated output, and a semiconductor laser transmitting enhanced output power.
2. Description of the Related Art
A semiconductor optical amplifier has the following advantages in comparison with other optical amplifiers.
First, a semiconductor optical amplifier can be formed smaller in size than other optical amplifiers.
Second, a semiconductor optical amplifier can be integrated with other functional devices.
Third, a semiconductor optical amplifier can be made with advanced functionality.
Thus, a semiconductor optical amplifier has been the subject of much research and development.
In particular, a waveguide type semiconductor optical amplifier has a function of transmitting data expressed in the form of a signal, together with a light. Hence, a waveguide type semiconductor optical amplifier is generally designed to include a waveguide which satisfies single-mode conditions or quasi single-mode conditions. This is because, if a signal light had a multi-mode, there would be caused problems that a signal light would be influenced by multi-mode dispersion, and that it would be difficult to efficiently apply a signal light to other optical waveguides or lens.
On the other hand, if a waveguide type semiconductor optical amplifier or a waveguide type laser diode is designed to satisfy a quasi single-mode, there would be caused a problem that had limited its characteristics. For instance, in a semiconductor amplifier, a width and a thickness of an active layer are limited by single-mode conditions. One of the simplest methods of improving a gain saturation level is to widen a width of a waveguide, for instance. However, this method is limited by the single-mode conditions as mentioned above, and hence, there is a limitation in increasing output power.
In order to avoid dependency on a polarized light, and avoid the limitation caused by the above-mentioned single-mode conditions, Japanese Unexamined Patent Publication No. 11-132798 (A) has suggested a semiconductor optical amplifier. The suggested semiconductor optical amplifier is an active MMI (Multi-Mode Interference) type one, and is designed to include a 1xc3x971-MMI waveguide in the vicinity of an output end for improving a saturated output level. By improving a saturated output level by 10 dB or more, the above-mentioned problem of gain saturation in the vicinity of an output end can be solved. However, there is caused another problem that gain saturation in a single-mode waveguide located in the vicinity of a MMI waveguide limits a saturated output level.
Japanese Unexamined Patent Publication No. 2000-323781 (A) has suggested a semiconductor optical amplifier and a semiconductor laser each of which includes a single-mode waveguide, and a multi-mode interference waveguide having a waveguide width greater than a width of the single-mode waveguide and optically connected to the multi-mode interference waveguide. By connecting the multi-mode interference waveguide having a greater width than that of the single-mode waveguide, to the single-mode waveguide, there is obtained the advantage which would be obtained by widening a waveguide width of a portion of the single-mode waveguide, and it would be possible to improve a gain saturation level (saturated output level) without changing single-mode conditions. As a result, it would be possible to provide the semiconductor optical amplifier with higher saturation output power and to provide the semiconductor laser with enhanced output power, and to improve COD level and spatial hole burning, prevent efficiency reduction caused by mode conversion loss, and enhance mode stability in the semiconductor optical amplifier or in the semiconductor laser.
However, the active MMI type optical amplifier suggested in Japanese Unexamined Patent Publication No. 11-132798 (A) is accompanied with a problem that gain saturation would be remarkable in a single-mode waveguide located in the vicinity of a MMI waveguide, if a saturated output is to be improved by 10 dB or more, and thus, a saturated output level is limited. An active MMI type semiconductor laser which is coated at a rear facet thereof with a film having a high reflectivity and at a front facet thereof with a film having a low reflectivity is accompanied with the same problem as mentioned above.
The semiconductor optical amplifier and the semiconductor laser suggested in Japanese Unexamined Patent Publication No. 2000-323781 (A) is designed to include a multi-mode interference waveguide region for widen a waveguide width as much as possible in order to reduce power consumption and improve a saturation level. This structure causes a significant difference in waveguide widths. Hence, the multi-mode interference waveguide region causes a problem that gain saturation appears remarkable for an intensive incident light, with the result that expected performances cannot be obtained.
Japanese Unexamined Patent Publication No. 11-68240 (A) has suggested a semiconductor optical amplifier which emits a single-mode light and has a waveguide structure including a multi-mode interference waveguide region.
Japanese Unexamined Patent Publication No. 11-68241 (A) has suggested a semiconductor laser which emits a single-mode light and has a waveguide structure including a multi-mode interference waveguide region.
However, the above-mentioned problems remain unsolved even in those Publications.
In view of the above-mentioned problems in the conventional semiconductor optical amplifiers and semiconductor laser, it is an object of the present invention to provide a semiconductor optical amplifier which is capable of improving a saturated output of an active MMI type optical amplifier, and producing high output power.
It is also an object of the present invention to provide a semiconductor laser which is capable of producing higher output power than that of an active MMI type semiconductor laser.
In one aspect of the present invention, there is provided a semiconductor optical amplifier including (a) a single-mode waveguide region which provides a single-mode to a guided light-wave, (b) a first multi-mode interference waveguide region which has a greater waveguide width than that of the single-mode waveguide region, is optically connected to the single-mode waveguide region, and provides a mode including a multi-mode, to the guided light-wave, and (c) a second multi-mode interference waveguide region which has a greater waveguide width than that of the first multi-mode interference waveguide region, is optically connected to the first multi-mode interference waveguide region, and provides a mode including a multi-mode, to the guided light-wave.
In accordance with the above-mentioned semiconductor optical amplifier, the first and second multi-mode interference waveguide regions are designed to have an increasing waveguide width. Hence, it would be possible to prevent deterioration in performances of the semiconductor optical amplifier caused by connection of waveguide regions to each other, and improve a saturated output level with both the single-mode characteristic and the dependency on polarized light being sustained.
It is preferable that each of the first and second multi-mode interference waveguide regions is formed as a 1xc3x971 multi-mode interference waveguide.
There is further provided a semiconductor optical amplifier including (a) a single-mode waveguide region which provides a single-mode to a guided light-wave, (b) a first multi-mode interference waveguide region which has a greater waveguide width than that of the single-mode waveguide region, is optically connected to the single-mode waveguide region, and provides a mode including a multi-mode, to the guided light-wave, and (c) a second multi-mode interference waveguide region which has a greater waveguide width than that of the first multi-mode interference waveguide region, is optically connected to the first multi-mode interference waveguide region, and provides modes including multi-modes, to the guided light-wave, wherein at least one of the first and second multi-mode waveguide regions is comprised of a plurality of such sub-regions that a sub-region located closer to an output end of the semiconductor optical amplifier has a greater waveguide width.
In accordance with the above-mentioned semiconductor optical amplifier, a multi-mode interference waveguide region includes the first and second multi-mode interference waveguide regions, the first and second multi-mode interference waveguide regions have different waveguide widths from each other, and the first and/or second multi-mode interference waveguide regions are(is) comprised of a plurality of such sub-regions that a sub-region located closer to an output end of the semiconductor optical amplifier has a greater waveguide width. This structure prevents deterioration in performances of the semiconductor optical amplifier caused by connections between the waveguide regions, and makes it possible to improve a saturated output level with both the single-mode characteristic and the dependency on polarized light being sustained.
It is preferable that the semiconductor optical amplifier further includes a second single-mode waveguide region or a quasi single-mode waveguide region, both optically connected to the second multi-mode interference waveguide region at its output end.
For instance, the second single-mode waveguide region or the quasi single-mode waveguide region may be formed of a single-mode waveguide, or consisted from 1xc3x971 multi-mode interference coupler (MMI), or first-order mode allowing waveguide.
In another aspect of the present invention, there is provided a semiconductor laser including (a) a single-mode waveguide region which is coated at a rear facet thereof with a high-reflection coating and at a front facet thereof with an anti-reflection coating, and provides a single-mode to a guided light-wave, (b) a first multi-mode interference waveguide region which has a greater waveguide width than that of the single-mode waveguide region, is optically connected to the single-mode waveguide region, and provides modes including multi-modes, to the guided light-wave, and (c) a second multi-mode interference waveguide region which has a greater waveguide width than that of the first multi-mode interference waveguide region, is optically connected to the first multi-mode interference waveguide region, and provides a mode including a multi-mode, to the guided light-wave.
In accordance with the above-mentioned semiconductor laser, the first and second multi-mode interference waveguide regions are designed to have an increasing waveguide width. Hence, it would be possible to prevent deterioration in performances of the semiconductor laser caused by connection of waveguide regions to each other, and increase output power in a single-mode.
There is further provided a semiconductor laser including (a) a single-mode waveguide region which is coated at a rear facet thereof with a high-reflection coating and at a front facet thereof with an anti-reflection coating, and provides a single-mode to a guided light-wave, (b) a first multi-mode interference waveguide region which has a greater waveguide width than that of the single-mode waveguide region, is optically connected to the single-mode waveguide region, and provides a mode including a multi-mode, to the guided light-wave, and (c) a second multi-mode interference waveguide region which has a greater waveguide width than that of the first multi-mode interference waveguide region, is optically connected to the first multi-mode interference waveguide region, and provides a mode including a multi-mode, to the guided light-wave, wherein at least one of the first and second multi-mode waveguide regions is comprised of a plurality of such sub-regions that a sub-region located closer to an output end of the semiconductor optical amplifier has a greater waveguide width.
In accordance with the above-mentioned semiconductor laser, a multi-mode interference waveguide region includes the first and second multi-mode interference waveguide regions, the first and second multi-mode interference waveguide regions have different waveguide widths from each other, and the first and/or second multi-mode interference waveguide regions are(is) comprised of a plurality of such sub-regions that a sub-region located closer to an output end of the semiconductor optical amplifier has a greater waveguide width. This structure prevents deterioration in performances of the semiconductor laser caused by connections between the waveguide regions, and makes it possible to increase output power in a single-mode.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.